The present invention relates to a static apparatus such as a transformer and a reactor and particularly to a static apparatus that cools the internal space of a winding assembly with coolant.
For a static apparatus such as a transformer and a reactor, the density of its heat generation caused by loss tends to increase, along with technical development for making its capacity greater and its size smaller. To cool such heat, a method of filling the tank of the static apparatus with a coolant is widely used.
For example, in the case of a transformer, a transformer assembly is housed in a tank and the tank is filled with a dielectric liquid coolant such as mineral oil, silicone liquid, vegetable oil, or synthetic ester oil so that the transformer assembly is immersed in the liquid. The transformer assembly is cooled by using cooling equipment such as a radiator and ribs or by circulating the liquid coolant through ducts formed between the tank wall surfaces and the transformer assembly. In the transformer assembly, a winding is the source of heat generation. A structure of ducts that are formed using insulation solids to allow the liquid coolant to flow from a section under the winding into a region surrounding the winding, while cooling the winding, flow out to a section above the winding is widely used. An internal coolant duct surrounding the winding is connected to coolant ducts provided in the sections of upper and lower parts that support and fix the winding from top and bottom.
For proper operation as the transformer, insulation must be ensured between each winding of a primary winding and a secondary winding or more windings, between electric conductors in the windings, between the windings and their cores, between the transformer tank and the windings, and the upper and lower end portions of the windings and their peripheral structures. The transformer is designed and manufactured to ensure dielectric strength in required specifications. In this process, it is most reasonable to develop a design to prevent a partial discharge from occurring and not to exceed upper-limit applied voltages in the required specifications. However, it is difficult to completely eliminate a possibility that a severe situation occurs with a high voltage temporarily in excess of the specifications, such as generation of a voltage higher than withstanding voltages required in specifications because of lightning strike or the like during use.
Therefore, taking such a severe situation into consideration, it is more preferable that the transformer has a structure in which a partial discharge, if occurs, is unlikely to lead to insulation breakdown. Generally, if a partial discharge has occurred in the lower end or upper end portion of the winding, the discharge progresses toward peripheral structures like parts fastening core, when the discharge progresses through the coolant ducts in the sections of the upper and lower parts supporting the winding and, in most cases, progresses through the coolant and along the surfaces of the insulation solids forming the coolant ducts. When the progressed discharge reaches the peripheral structures like the parts fastening core, it results in insulation breakdown. To cause insulation breakdown, the longer the streamer length, the larger energy causing the discharge is needed.
For example, in Japanese Unexamined Patent Application Publication No. 2013-65762, a method is disclosed that divides the space of a coolant duct into small partitions by insulation solids, thus reducing the probability of existence of a weak point in terms of insulation within one space.